Optimized power lines

An optimized "electricity highway" that transports electrical energy reliably over long distances – preferably still underground? Today’s technology will not do and research in new DC transmission technologies is important.


In the past, Germany’s electricity was mainly produced in large power plants located close to the main consumers in urban areas. Nowadays electrical power systems (and energy distribution systems in general) are facing one of the largest technological changes ever. Due to the increasing renewable energy percentage in the energy mix, large amounts of electrical energy have to be transported over long distances. Large offshore wind parks in the North Sea are more efficient than wind farms in southern Germany, where many of the densely populated areas with high electricity consumption are located.

With an “electricity highway”, an optimised transmission grid which can efficiently and reliably transport the electrical energy over long distances, it is much easier to balance electricity generation and demand than relying on regional grids. Less wind farms and solar plants need to be installed to ensure a reliable sustainable power supply.

However, building new transmission lines is often difficult due to opposition from citizens in the affected areas. Underground cables could be an alternative to overhead lines. Today’s AC technology, though, is not suited for an efficient long-distance transmission underground. Therefore, new technologies have to be developed in order to secure our future energy supply.


• Developing technological components, systems and operating strategies for an efficient power transmission (AC and DC);

• Developing concepts for stabilising the electrical power system supplied by an increasing amount of renewable energy from volatile, decentralised sources, facing new consumers such as electric mobility and electricity-based heat supply for the building sector;

• Optimising the energy management so that operating resources are used more efficiently and economically.

Selected projects and test sides

Fermi Level Engineering of Antiferroelectric Materials for Energy Storage and Insulation Systems

The LOEWE project “FLAME – Fermi Level Engineering of Antiferroelectric Materials for Energy Storage and Insulation Systems” investigates how the properties of functional materials can be adjusted via their electronic structure. Twelve research groups from the fields of materials science, geosciences, chemistry, electrical engineering and information technology are collaborating to develop lead-free antiferroelectrics for capacitors with high energy and power density and for high-voltage insulators. These enable a more efficient conversion and transmission of electrical energy from renewable sources and in electromobility. Tongji University in Shanghai is also involved in the project.

The research approach, which can be transferred to other materials and fields of application, is based on adjusting optimised electronic structures (“Fermi Level Engineering”), which can be predicted with computer simulations and implemented experimentally. This enables a precise adjustment of the properties along with shorter development periods.

The project is funded by the State of Hesse as part of the 11th tier of the LOEWE initiative from January 2019 to December 2022.

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A field experiment facility was built at TU Darmstadt in 2013 to study the ampacity of directly buried energy cable systems under environmental conditions. The test field was established by a collaborative research alliance of the Department of Geothermal Science and Technology and the High Voltage Laboratories at TU Darmstadt as well as a Bavarian distribution grid operator.

This experimental set-up allows for investigations on various cables and cable arrangements in different bedding materials and soils under natural conditions. The overall dimension of the test site is 14 m by 6 m, divided into four sections of 3.5 m length that are hydraulically and thermally decoupled from each other. In these sections, cables for medium voltage and low voltage are routed through silt, sand, clay and a liquid-soil bedding. The changes of the thermal and hydraulic conditions within the test site are monitored by numerous sensors.

When electrical energy is transmitted via buried cables, electrical losses are dissipated to the environment as heat resulting in a temperature increase of the cable conductor. As the temperature is limited and directly proportional to the current, the so-called thermal current rating represents an important limit of the cable run capacity.

At present, the cable ampacity ratings are established based on standardised consumption patterns and conservative assumptions regarding the thermal properties of the bedding. Thereby, values below the actual limit are derived. Not considered is the influence of natural water content changes in the surrounding soil on the thermal properties of the bedding..

By adopting the ratings to the actual thermal limit, the need for expanding the ground cable infrastructures is reduced significantly, which can help lower the costs of the German energy transition. This is of particular interest as the need for expansion usually arises because of dispersed power injection into distribution networks.

The results of these field studies, laboratory investigations and numerical models lay the ground for a calculation tool that in future will show grid operators the possible load capacity in real time.

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An on-site test laboratory for direct-buried gas-insulated transmission lines was built. In this laboratory, transmission lines for 500-kV high voltage direct current (HVDC-GIL) can be tested under realistic operating conditions in long-term experiments of at least one year.

This new technology is a compact alternative to overhead power lines and underground cables for the transport of electrical energy. It can significantly contribute to ensuring a stable energy supply based on renewable energies by transporting electricity between North and South Germany for instance.

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